The present invention generally concerns radiation detection devices and monitoring methods, and more particularly concerns dosimetry devices and methods for personal and area uses.
Monitoring and measuring the effects of radiation upon biological organisms and microelectronic circuits are. critical concerns in the nuclear energy applications (e.g., nuclear power stations, nuclear medicine, nuclear weapons industry, nuclear propulsion systems, etc.) and space exploration technologies, especially in the low dose natural radiation environment of space. Modern microelectronic components in particular are known to be subject to a wide variety of detrimental radiation induced phenomena from sporadic operation to total failure.
Various dosimetry and radiation detection devices are known and utilized in the art for measuring and predicting the effects of a radiation field upon biological tissue in particular and microelectronics in general. These devices include thermoluminesent dosimeters (TLD), ionization chambers, and Geiger tubes. These devices, however, are generally limited in that they effectively only provide a total dose measurement from a radiation field. To accurately predict the risk involved in exposing an organism or microelectronic circuit to some complex radiation environment, it is imperative to analyze the qualitative effects of the various ionizing particles in the complex radiation field. The conventional dosimetry devices generally have not effectively accomplished this goal.
In the relatively recent past, the art has turned its attention to metal oxide semiconductor devices as radiation detectors. When a metal oxide semiconductor device is irradiated, charge builds up in the oxide layer. These charges can be measured and this has proven a useful dosimetry principle. See, for example, U.S. Pat. No. 4,517,464 to Heath et al.; U.S. Pat. No. 4,859,853 to Kronenbero: U.S. Pat. No. 4,757,202 to East; U.S. Pat. No. 4,608,655 to Wolf et al.: and the scholarly publication in IEEE Transactions on Nuclear Science, Volume NS-33, No. 6, December 1986, entitled "PMOS Dosimeters: Long-term Annealing and Neutron Response."
The principles and operation of the solid-state detectors are known and understood by those skilled in the art. However, these solid-state particle detectors are again generally only useful for a total quantitative analysis of a radiation field. For example, with a typical conventional silicon surface barrier detector consisting of a relatively large (e.g., 25 mm.sup.2 .times.100 micron) biased p-n junction, circuitry is generally provided to measure and record the total amount of charge collected as a result of the junction being traversed by charged ionizing particles. The voltage pulse generated is normally sent to a preamplifier used in pulse height analysis. The preamplifier integrates the voltage pulse over time and generates a signal which is proportional to the total charge collected across the junction. This signal is then sent to a pulse height analyzer for quantitative analysis. For a full discussion of the operation of solid-state detectors such as the silicon surface barrier detector and pulse height analyzers, see the work by G. F. Knoll, "Radiation Detection and Measurements," John Wiley & Sons, New York, N.Y., 1989.
The junctions of conventional solid-state particle detectors are relatively large, as discussed above. These junctions do not simulate a microstructure as tiny as a biological cell or the p-n junction of a microelectronic semiconductor. Thus the conventional solid-state detector generally cannot qualitatively analyze an incident radiation field on a micron level to determine precisely what type of ionizing particle is producing the induced voltage within the junction. Such detectors generate a total voltage from the total incident radiation field and circuitry is known in the art for measuring this total voltage. However, this analysis provides very little information on the effects of radiation on the critical microstructure level.
Conventional solid-state particle detectors generally do not provide adequate means for measuring the energy of specific particles that make up the complex radiation environment, nor for predicting the detrimental effects the constituent elements of a complex radiation field will have upon a microvolume of tissue or a microelectronic junction. For instance, a conventional solid-state dosimeter will yield a fairly accurate estimate o a total dose from an incident complex radiation field, but cannot separate the contributions from the individual types of radiation that make up the complex radiation field. Such information is essential to estimating the risk to a biological organism or microelectronic circuit. The probability of whether an incident ionizing particle will result, for example, in a genetic chromosome mutation in a biological cell or an upset event in a microelectronic junction depends upon the amount of energy the incident particle deposits within the microstructure biological cell or electronic junction. Conventional dosimetry devices generally do not provide this qualitative information.
Fairly recently, significant work has been done in predicting single event phenomena in modern micro-electronic circuits. Modern microelectronic circuits are subject to a variety of single event phenomena, or "upsets," caused by incident radiation. Such circuits may, for example, consist of a particularly arranged array of p-n junctions. Typically, operation of the circuit involves the selected storage and switching of various electrical charges (or logic states) at the given junction When ionizing particles traverse these junctions, they cause sudden undesirable swings in the bias (i.e., the electrical charge) across the junction. The generation of a relatively large charge at these critical locations on the die of a microelectronic chip can alter the electrical condition, and hence the logic state, of an element, which is referred to as an "upset."
Traversal of certain junctions on a circuit by an ionizing particle would be sufficient to induce changes in the electrical state in any circuit element, unless they are specifically hardened against such changes. Hardening adds to cost while sometimes adversely affecting performance or conflicting with design constraints, e.g., weight limitations. Upset phenomena is a significant concern in the microelectronic field, especially in the space technology area where these types of microelectric circuits are constantly exposed to complex low level radiation fields. Analysis and prediction of this upset phenomena is essential to ensuring the reliability and sustained operational ability of microelectric circuits in space. The extreme financial losses associated with the failure of space based systems are well known.
Reverse biased junctions are essential components of microelectronic circuits. It is known that various types of radiation induce single event upsets at these junctions. These include heavy ions, protons, neutrons, alphas, and gammas. The amount of charge which must be collected across the junction to upset the circuit element is called the critical charge. In order to predict the single event upset (SEU) rates in microelectronic circuits, it was necessary to first determine the dimensions of the sensitive volumes of the particular junctions. The sensitive volume is generally defined to be that region about the junction within which the charges generated by the traversing radiation particles are efficiently collected at the junction. For the conventional surface barrier type detector discussed above, the sensitive volume is virtually the entire slab of silicon within the detector.
An important advance in predicting SEU rates of microelectronic circuits involved the ability to effectively measure and/or confirm the sensitive volumes of the micron sized junctions in solid-state electronic components (e.g., semiconductors). For a complete discussion of this aspect of the art, see the scholarly article presented by P.J. McNulty at the IEEE Nuclear and Space Radiation Effects Conference in Reno, Nev., Jul. 16, 1990, entitled "Predicting Single Event Phenomena in Natural Space Environments."
In theory, the "perfect" solid-state detector would approximate the size of a single biological cell or p-n junction of a microelectronic circuit. In this case, the charge induced in the detector would necessarily be from the same type and amount of radiation which would cause damage to a biological cell or cause an upset in a p-n junction of that size in a microelectronic circuit. Hence, the relatively large junctions of prior art devices have generally failed to achieve such heretofore theoretically advantageous results.